UK Patent Specification No. 1,427,647 and U.S. Pat. No. 3,676,288 describe the application to, or incorporation in, a fibrous web of non-expanded microspheres by means of a binder, for example, a polyacrylonitrile latex. As the binder is dried and crosslinked, the spheres are attached to the fibrous web and expanded.
Such webs are extremely suitable as a reinforcement for all kinds of cured synthetic plastics materials, such as polyester resin or epoxy resin. In fact, the advantage in using the expanded microspheres in combination with the fibrous web is that the incorporation of the extremely light spheres in the resin is considerably simplified. The use of the microspheres results in a considerable saving of resin and glass fiber, and the mechanical properties of the product reinforced with the web are at least maintained. Rigidity and impact resistance are even improved, and the thermal insulation capacity is enhanced too.
In the practice of this method, it was found that the displacement of the air in the fibrous web by the liquid resin can be accelerated by perforating the carrier material with holes of approximately 1.5 mm spaced short distances apart, for example 5-15 mm. It has been found, that the impregnation is sensitive to changes in procedure. If, for example, one waits longer, it may occur that certain parts of the web come to float, as a result of which the resin sinks back into the perforations and an air bubble or air passages can be formed at these locations. This means that upon completion a potential local weakening may be formed.